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Abstract

A simple solvothermal approach is explored to prepare Bi2−xMoxSe3 nanostructures by employing N,N-dimethylformamide (DMF) as the solvent. Mo plays an important role in the assembly
of the Bi2−xMoxSe3 nanostructures from nanoplates to nanoflowers. Structural and morphological studies
indicate that the resulting products are large specific surface area single-crystalline
Bi2−xMoxSe3 nanoflowers self-assembled from thin nanoplates during the reaction process. The
absorption properties of the as-prepared samples are investigated with Rhodamine B
(RhB) as dye, and it is found that the Bi1.85Mo0.15Se3 nanoflowers show an optimal adsorption capacity, implying that Mo doping not only
changes the morphologies of the nanostructures but also enhances their absorption
behaviors.

Keywords:

Background

Water pollution has now become an urgent problem owing to the rapidly growing global
industrial process [1,2]. Public health and social economies are threatened by various organic dye pollutants
from textile industries [3]. A variety of methods have been introduced to remove dyes from wastewaters, such
as membrane filtration [4], flotation [5,6], solvent extraction [7], chemical oxidation [8,9], adsorption [10,11], and photocatalytic degradation [12,13]. Among these methods, adsorption has been proved to be an effective way for wastewater
treatment in terms of simplicity of design, user-friendly control, and insensitivity
to toxic substances. Dye removal from industrial wastewaters by adsorption techniques
has been widely concerned and researched in recent years [10-15]. Activated carbon is considered one of the best adsorbents for the removal of organic
contaminants, but activated carbon is too expensive to use widely in practical applications
[16]. Therefore, the development of low-cost, high-efficiency, renewable, and eco-friendly
materials as absorbent for the removal of dyes has attracted more and more interests.
Recently, many kinds of materials such as SnS2 nanosheets [15], WO3 nanorods [17], Cu2O nanocrystals [18,19], and other highly adsorbent materials have been investigated.

Bismuth selenide (Bi2Se3) nanostructures have been extensively studied due to their unique properties and
promising applications in the fields of optical recording systems, laser materials,
optical filters, sensors, solar cells, strain gauges, electromechanical and thermoelectric
devices, and topological insulators [20-23]. During the past few years, the preparation and application of doped Bi2Se3 have been extensively investigated [24-27]. In addition, due to the high surface state and unique optical or electrical properties
[28], Bi2Se3 can also be applied in the fields of visible-light photocatalytic degradation [27,29]. For example, Bi2Se3-TiO2 complex nanobelts [30] and S-doped BiSe [31] show excellent visible-light photocatalytic degradation performance. However, to
our knowledge, there is no report on the absorption properties of Bi2Se3 nanostructures, especially the systematic study of the Mo doping-enhanced absorption
behavior of Bi2Se3 nanostructures.

In this work, we synthesized self-assembled Mo-doped Bi2Se3 nanoflowers by a simple solvothermal route. We find that the absorption behavior
of Bi2−xMoxSe3 on Rhodamine B (RhB) varies as a function of Mo content and reaches its highest absorption
capacity with 15% Mo doping.

Methods

Preparation of Bi2−xMoxSe3

All of the chemical reagents used in this experiment are of analytical grade and used
without further purification. Bi2−xMoxSe3 (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.15) is obtained by a simple solvothermal method.
In a typical Bi2−xMoxSe3 (x = 0.15) synthesis, 0.85 mmol of Bi(NO3)3·5H2O and 0.15 mmol of (NH4)6Mo7O24·4H2O are added to 18 ml of N,N-dimethylformamide under vigorous stirring to form a homogeneous solution. Then additional
ammonia is added to the above solution to adjust the pH value to 9 to 10 under continuous
stirring. After that, Se powder and Na2SO3 are added to the above solution under magnetic stirring. The final solution is transferred
into a Teflon-lined autoclave (25-ml capacity), kept at 160°C for 20 h, and cooled
to room temperature under ambient conditions. The products are finally washed several
times with ethanol and distilled water, followed by drying at 80°C for 12 h under
vacuum. For comparison, we also synthesized Bi2−xMoxSe3 samples with different Mo contents (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.15), which are labeled as samples A, B, C, D,
E, and F, respectively.

Dye adsorption experiments

The adsorption activities of the as-prepared products are investigated using RhB as
dyes. In each experiment, 0.08 g of adsorbent was added to 50 ml of a 10-mg/l RhB
solution. Under constant stirring in the dark, about 6 ml of the mixture solution
is taken out at intervals and centrifuged to separate solid particles for analysis.
After centrifugation, the adsorption behavior is investigated.

Sample characterization

The phase composition and crystallographic structure of the as-prepared samples are
examined by X-ray diffraction (XRD) technique with Cu Kα irradiation. The sizes and
morphologies of the products are investigated using a field emission scanning electron
microscope (FESEM; S-4800, Hitachi, Minato-ku, Tokyo, Japan). The dye adsorption behavior
is measured with a UV-visible (UV–vis) spectrum (Lambda 900, PerkinElmer Instruments,
Branford, CT, USA).

Results and discussion

Structure and morphology

The as-prepared samples are examined by XRD techniques, and the XRD patterns of samples
A to F are shown in Figure 1. All the peaks in the patterns can be indexed according to the power diffraction
card of hexagonal Bi2Se3 (no. 33-0214), and no impurity phase related to the Mo complex could be found. The
diffraction peaks shift to higher angles with the increase of Mo6+ content from samples A to F, indicating that Mo6+ has been incorporated in the Bi2Se3 lattice, and the lattice parameter gets smaller with the increase of Mo6+. This is understandable considering the fact that the ionic radius of Mo6+ (0.065 nm) [24] is smaller than that of Bi3+ (0.103 nm) [25].

The morphology and size of the as-synthesized products are characterized by FESEM
observations (Figure 2). The low-magnification FESEM image in Figure 2a shows that a large number of platelike nanostructures are randomly dispersed on
the surface of the substrate. Comparatively, a perfect hexagonal morphology for Bi2Se3 is observed from the image. A magnified FESEM image (Figure 2b) shows that the width of the nanosheets is in the range of 100 to 400 nm with a
thickness of about 10 to 30 nm. The doping of Mo changes the morphologies of the nanosheets
greatly. The low-magnified FESEM image (Figure 2c) demonstrates that the typical product of Bi1.85Mo0.15Se3 consists of a large quantity of uniform flowerlike nanospheres. The average diameter
of flowerlike nanospheres is about 100 to 200 nm, and they are made up of curved nanoplates
with an average thickness of 5 to 10 nm as shown in the high-magnification FESEM image
(Figure 2d).

To confirm the structure, crystallinity, and details of the flowerlike nanospheres,
high-resolution transmission electron microscopy (HRTEM) techniques are employed.
A representative HRTEM image taken from the edge of a Bi1.85Mo0.15Se3 nanoflower is shown in Figure 3a, which clearly indicates that nanoflowers contain a perfectly periodic arrangement
with an interplanar distance of a = 0.356 nm, which is smaller than that of bulk anatase (a = 0.413 nm) crystal.

The chemical composition of Bi2−xMoxSe3 was determined by energy-dispersive X-ray analysis (EDXA) attached to the FESEM.
In Figure 3b, the EDXA spectrum of the Bi1.85Mo0.15Se3 nanosheets shows that the nanosheets contain only Mo, Bi, and Se without any trace
of by-products.

From the FESEM observations, we can conclude that the Mo concentration influences
the morphologies of the nanoplates greatly. In order to understand the role of Mo
in the evolution process of Bi2−xMoxSe3 from nanoplates to nanoflowers, Bi2−xMoxSe3 samples with varied x values are synthesized and studied. With the increase of the x value (Mo concentration), the nanostructures gradually change from hexagonal nanosheets
to smaller-sized hexagonal nanosheets and finally to flowerlike spheres, combined
with a size change. For example, when no Mo is contained, the sizes of the nanosheet
are about 100 to 500 nm in width and 20 to 30 nm in thickness, as shown in Figure 4 (a-1 and a-2). With increasing amounts of Mo, morphologies of the as-synthesized
Bi2−xMoxSe3 products change from nanosheets to nanoflowers (Figure 4 (a-1 to f-2). From Figure 4 (b-1 to d-2), we can see that the products are still composed of nanosheets, but
the sizes of Bi2Se3 have become smaller. When the Mo concentration increased up to 15% (Figure 4 (f-1 and f-2), regular flowerlike spheres consisting of thin nanoplates were formed.
The average diameter of nanoflowers is about 100 to 200 nm, and they are made up of
curved nanoplates with an average thickness of 5 to 10 nm as shown in the magnified
image. With the increase of Mo contents in Bi2−xMoxSe3, the diameter of the products was found to be lower than that of pure Bi2Se3. So we believe that Mo is the main driving force for the formation of a flowerlike
structure.

A diagram of the formation mechanism of nanoplates and nanoflowers of Bi2−xMoxSe3 is presented in Figure 5. When no Mo is contained, tiny clusters of Bi2Se3 nanosheets are first generated upon heating and then enriched to assemble into bigger
nanosheets. However, with the increase of Mo concentration in Bi2−xMoxSe3, nanosheets assemble into nanoflowers.

Figure 5.Schematic diagram showing the growth mechanism of nanosheets and nanoflowers of Bi2−xMoxSe3.

Adsorption ability of Bi2−xMoxSe3

To investigate the potential application of the as-synthesized Bi2−xMoxSe3 nanocrystals and their relationship with the amount of Mo in Bi2−xMoxSe3, we study the adsorption ability of Bi2−xMoxSe3 using RhB as dyes. The experiments are carried out with Bi2−xMoxSe3 dispersed in the solution of RhB in the dark several times with constant stirring.
After centrifugation, the UV–vis absorption of the supernatant was measured and the
characteristic absorption of RhB at about 553 nm was selected to estimate the adsorption
process. Figure 6 shows the UV–vis adsorption spectra of RhB as a function of time using the as-prepared
Bi1.99Mo0.01Se3 as adsorbent. From Figure 6, we can see that the intensity of the absorption spectra gradually decreases and
nearly disappears within 60 min; at the same time, the solution becomes colorless
when observed with the naked eye.

Figure 7 shows the variation in RhB concentration with the adsorption time over different
adsorbents. When there is no adsorbent, the concentration of the RhB solution remains
the same with its original state for up to 60 min, which demonstrates that RhB is
stable under the experimental conditions. Pure Bi2Se3 shows a weak adsorption activity. After 60 min of absorption, only 20% of RhB is
removed from the pure Bi2Se3 sample. The adsorption activities are strengthened with the increase of Mo contents
in Bi2Se3. Bi1.85Mo0.15Se3 has a maximum adsorption behavior, and nearly 100% of the RhB dyes are removed in
20 min. All of this clearly shows that the doping of Mo in Bi2Se3 is an efficient way to enhance its adsorption activity. The results indicate that
the as-synthesized Bi2−xMoxSe3 might possess a profound application in the fields of treatment of dye-polluted wastewater.

Conclusions

In summary, Bi2−xMoxSe3 nanomaterials were prepared by a solvothermal approach, and different morphologies
of Bi2−xMoxSe3 have been obtained. The doping concentration of Mo plays an important role in controlling
both the morphologies of Bi2−xMoxSe3 nanostructures and their absorption behavior. The sample with the best absorption
behavior is that with 15% Mo concentration. We believe that the study of dye absorption
behavior brings a new application realm for Bi2Se3 nanostructures.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

MZ, XM, and JL designed the experiments. MZ and XM performed the experiments. MZ,
FW, and YF analyzed the data. MZ made the figures. MZ and XM wrote the manuscript.
All authors read and approved the final manuscript.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (grant
nos. 11104250 and 61274099), the Science Technology Department of Zhejiang Province
(grant no. 2012C21007), and the Zhejiang innovative team (2011R50012).